1. Field of the Invention
The invention relates to a gear transmission with stepless adjustable translation ratio for converting rotational speeds and torques between two shafts.
2. Brief Description of the Background of the Invention including Prior Art
Such converters are employed in the complete drive technology, in particular with machine tools and drives for vehicles.
Motors depend in their mechanical power on the rotation speed. Motors operate usually in their most economic region of rotation speed under favorable energy use or in the region of maximum delivered power. In contrast the driven machines are to exhibit different rotation speeds depending on their use or, respectively, their operating state in each case. The rotation speeds of drive unit and driven machine are adapted to each other by gear change boxes with corresponding transmission ratios. The loss in motor power can be kept small upon optimum setting of the transmission ratio. An optimum transfer of the rotary motion is possible under small power loss with gear transmissions having steplessly adjustable transmission ratios with a corresponding degree of effectiveness of the arrangement.
Mechanical, electrical and hydraulic converters are distinguished with such stepless rotation speed converters corresponding to their operating principle.
Mechanical systems for rotation speed conversion have become known as friction gear transmissions or traction mechanism transmissions. The motion (torque, rotation speed) is transferred in friction gear transmissions by frictional engagement at rotation symmetrical bodies with a steplessly changeable effective diameter or, respectively, effective circumference. The required friction between the friction wheels can only be achieved by large press-on forces. The large press-on forces require however high loads of material at the two contract points or, respectively, contact lines and represent a substantial load for the bearings and the drive shafts. In addition, friction wheel gear transmissions are associated with a substantial slippage, which slippage again results in a decreased degree of effectiveness of the arrangement.
Traction mechanism transmissions work as chain transmissions or belt transmissions with the drive members and driven members of a pair of cone disks, wherein the chain disks are endlessly surrounded by a chain or, respectively, by a belt. They work according to the principle of the stepless adjustment of the operating diameter of the cone disks, wherein at least one of the cone disks is axially adjustable. Belt drives however are only employed for smaller powers, chain drives are preferably employed with stationary machines.
The hydrostatic variable speed gears from a combination of a vane pump and vane motor are standing among the hydraulic transmissions with steplessly changeable rotation speed, wherein a pump transports a liquid into the motor and wherein the rotation speed of the vane motor (driven part) depends on the transport volume or, respectively, the gulp volume of the motor. The liquid flow is used for force transmission in hydrodynamic rotation speed converters with a pump and a turbine wheel. The hydraulic transmissions are to a large extent associated with slippage and require high expenditures.
Similarly electrical stepless transmissions are only suitable for special application purposes because of the not inconsiderable plant expenditures and therefore are not to be considered further in this context.
Gear transmission with overrunning clutches also belong to the mechanical stepless rotation speed converters and are frequently designated as ratchet gear mechanism, which transforms a uniform rotary motion of a drive shaft into periodical oscillations or non-uniform rotary motions. The periodical oscillations or non-uniform rotary motions are transformed into a uniformly directed rotary motion of the driven shaft in the region of the velocity peak by way of a free wheel clutch. Coupled gears or also curved gears in multiple presence, which are operating phase shifted relative to each other, are employed on the drive shaft in order to decrease the nonuniformity of the rotary motion and in order to achieve nearly constant translation ratios. A value more or less densely varying around a middle value is obtained for the angular speed of the driven shaft, wherein the variations become smaller the larger the number of the individual gears disposed next to each other and operating with phase shift.
The still present nonuniformity of the rotary motion can cause substantial load peaks and thereby causes a high wear and a low lifetime of the transmission gear. The ratchet gear transmissions, which have become known therefore are constructed only for small mechanical powers.
Transmission gears with intermittently operating and driving members have been proposed, wherein these gears strive for a principle uniformity of the transmission ratio. The results of these solutions however are either not satisfactory, not sufficient wear resistant or they require a high constructive expenditure.
A variation of a ratchet gear mechanism is proposed in the German patent 474,205, wherein the rotary motion of the drive shaft is transformed by way of several curve disks disposed on the driving shaft into a back and forth motion of push rods and into a back and forth motion of gear racks by intermediate positioning of a variable translation device, wherein the gear racks in the following successively operate on the driven shaft through gear wheels and limited slip couplings (free engine clutches, overriding clutches) and drive the driven shaft. The variable transmission device comprises a two armed lever with adjustable hinge point for changing of the in each case effective lever lengths and therewith also changing the size of the transmission ratio. The gear racks move with an uniform speed during the force transmission phase and thus can drive the driven shaft with a uniform angular speed.
The motion transfer from the drive shaft in the driven shaft is performed in the transmission gear system according to U.S. Pat. No. 4,565,105 through cam disks circulating with the driving shaft and sampling members or scanning members, which transform the rotary motion in each case into a translation motion, wherein the translation motion also is transferred in each case to a gear rack/gear wheel part transmission through in each case a rotatably supported lever with adjustable rotation axis and is converted into a rotary motion of the driven shaft by intermediate positioning of in each case a free wheel clutch.
A substantial deficiency of these transmission gear systems comprises that the wear appearances occur with the transformation of the translational motion of the gear rack into the rotary motion of the gear wheel, wherein the wear appearances are caused in particular by the sliding friction effects, are of substantial size especially in continuous operation and thus can substantially interfere with the reliability of the overall gear transmission. The wear appearances concerning in particular those regions of the gearing of gear rack and gear wheel, which regions of gearing are subjected more frequently to a particularly high mechanical load in continuous operation as compared to other regions during periodical loads. The non-uniform distribution of the material abrasion by wear, based on the nonuniform loading of the available stroke distance of the toothed rack, is further increased, so that in the case of a decreased stroke (decrease of the length of the stroke distance) based on an enlargement of the translation ratio setting, the force to be transferred still increases under the precondition of a constant transfer power.
The strong and in particular nonuniform wear at the function determining contact faces of the gearing entails deviations from the desired relative motion of the drive means, which in turn are the cause for a rapidly progressing wear process.
In addition there is generated a nonuniformity in the running of the gear transmission with the frequency of the gearing teeth engagement, because flank regions of each tooth wear off nonuniformity, wherein the wear processes are most pronounced at the foot of the tooth.
The recited disadvantages decrease the reliability and the operational life of such gear transmission systems substantially.